those are excellent points, but like you said you have a feeling which helps to motivate. now, if you DIDN'T take the tab of amphetamine, caffeine or whatever, that motivation may or may not be there and presumably if it(motivation) is not there, this can hinder your workout. so theoretically if you can forsee a lack of motivation for your next work out, popping a tab of ritalin can increase the intensity of your workout. granted you may not lift heavier, but you may do a few extra exercises being that you feel more energetic while amphetamine, caffeine etc. circulates through your blood. also if a person is of the mindset that he can lift heavier, than it is likely that he will. i'm sure everyone here has experienced how powerful a force the mind can be in weight lifting.